Liquid crystal displays (LCDs) have been widely used in various portable information products such as notebooks, personal digital assistants (PDAs), and video cameras, because of its portability, low power consumption, and low radiation. A typical LCD includes an LCD panel, a backlight for illuminating the LCD panel, a backlight control circuit for controlling the backlight, and a power circuit for providing operation voltages to the LCD panel and the backlight control circuit.
Referring to FIG. 3, one such power circuit 10 includes a first rectifying and filtering circuit 11, a PFC circuit 12, a protection circuit 13, an inverter circuit 14 and a second rectifying and filtering circuit 15.
The first rectifying and filtering circuit 11 includes two inputs 111, 112 configured to receive an external alternating current (AC) voltage, such as a 220V AC voltage, a full-bridge rectifying circuit 110 configured to convert the 220V AC voltage to a first direct current (DC) voltage, a filter capacitor 114 configured to stabilize the first DC voltage, and a first output 113 configured to output the first DC voltage. Two inputs of the full-bridge rectifying circuit 110 serve as the two inputs 111, 112. A positive output of the full-bridge rectifying circuit 110 serves as the first output 113. A negative output of the full-bridge rectifying circuit 110 is grounded. The filter capacitor 114 is connected between the first output 113 and ground.
The PFC circuit 12 includes a first inductor 121, a first diode 122, a first transistor 123, a PFC chip 124, and a first storage capacitor 125. The PFC chip 124 includes a control terminal 1242 and a ground terminal 1244. A terminal of the first inductor 121 is connected to the first output 113. Another terminal of the first inductor 121 is connected to a drain electrode of the first transistor 123, and is connected to ground via the positive terminal of the first diode 122, the negative terminal of the first diode 122, and the storage capacitor 125 in series. A gate electrode of the first transistor 123 is connected to the control terminal 2242 of the PFC chip 1242. A source electrode of the first transistor 123 is connected to ground.
The inverter circuit 14 includes a pulse width modulation (PWM) chip 141, a second transistor 142, and a transformer 143. The PWM chip 141 includes a pulse output 1411 configured to output a pulse signal. The transformer 143 includes a primary winding 1431 and a secondary winding 1432.
The pulse output 1411 is connected to a gate electrode of the second transistor 142 for switching on or switching off the second transistor 142. A terminal of the primary winding 1431 is connected to the negative terminal of the first diode 122. Another terminal of the primary winding 1431 is connected to ground via a drain electrode and a source electrode of the second transistor 142. The secondary winding 1432 is connected to the second rectifying and filtering circuit 15.
The second rectifying and filtering circuit 15 includes a second inductor 151, a second diode 152, a second storage capacitor 153, a second filter capacitor 154, and a voltage output 155. A terminal of the secondary winding 2432 is connected to a negative terminal of the second diode 152. The other terminal of the secondary winding 1432 is connected to ground via the second storage capacitor 153, and is connected to the voltage output 155 via the second inductor 151. The second filter capacitor 154 is connected between the voltage output 155 and ground. A positive terminal of the second diode 152 is grounded. The second filter capacitor 154 is connected between the voltage output 155 and ground.
The external AC voltage U1 is provided to the two inputs 111, 112 of the first rectifying and filtering circuit 11. The external AC voltage U1 is a sine wave voltage and a current I1 between the two inputs 111, 112 is a triangle wave current. The external AC voltage U1 is transformed into the first DC voltage U2 and is provided to the PFC circuit 12. The first DC voltage U2 is a pulse wave and a current I2 thereof is a triangle wave current.
The PFC circuit 12 is configured to adjust the triangle wave current I2 to be similar to the waveform of the first DC voltage U2, and synchronize phases of the first DC voltage U2 and the corresponding current I2. When the first transistor 123 is switched on in a first period t1, the first DC voltage U2 is grounded via the first inductor 121. Thus a gradually increased current I3 is generated in phase with the first DC voltage U2. When the first transistor 123 is switched off in a second period t2, a gradually decreased current I3 is generated in phase with the first DC voltage U2. The first storage capacitor 125 is charged via the current I3 via the first diode 124 in order to stabilize the first DC voltage U2.
The first DC voltage U2 is then provided to the primary winding 141. The PWM chip 141 generates the pulse signal for switching on or switching off the transistor 142. When the transistor 142 is switched on, a gradually increased current flows through the primary winding 1431 when the first DC voltage U2 is connected to ground via the primary winding 1431 and the second transistor 142. When the transistor 142 is switched off, the energy stored in the primary winding 1431 is discharged via the protection circuit 13. Thus current flowing through the primary winding 1431 is gradually decreased.
The secondary winding 1432 induces the current flowing through the primary winding 1431 and generates a second AC voltage across the secondary winding 1432. The second rectifying and filtering circuit 15 transforms the second AC voltage into a second DC voltage and provides the second DC voltage to a load circuit (not shown) via the voltage output 155.
However, the power circuit 10 includes both the PWM chip 141 and the PFC chip 124. Because operation frequencies of the PWM chip 141 and the PFC chip 124 are different and respectively fixed, an interference is generated between the two chips. Therefore, noise of the power circuit 10 is generated by the interference.
It is desired to provide a new power circuit which can overcome the above-described deficiency.